Discharge velocity control for pneumatic lifts



Dec. 21, 1954 J. NEWMAN DISCHARGE VELOCITY CONTROL FOR PNEUMATIC LIFTSFiled May 1, 1952 T arr 64s 1- -J p u a W m A... T M N W n I m 4 V -L mH. w .m A v B- U DISCHARGE VELOCITY CONTROL FOR PNEUMATIC LIFTSApplication May 1, 1952, Serial No. 285,453

7 Claims. (Cl. 302-59) This invention relates to the pneumatic elevationof granular contact material through an elongated lift pipe,particularly as applied to hydrocarbon conversions or other processeswhich involve a continuous circulation of granular contact material,such as beads or pellets of catalytic material, having an averageparticle size of about 14 mesh, or larger.

Specifically, the invention relates to a method for reducing thedischarge velocity of such contact material as it discharges from theupper end of the lift pipe into the usual disengaging vessel, in orderthat the contact material may be disengaged from the lift gas bycomplete gravitational deceleration within a minimum vertical distanceand permitted to fall freely to one or more collecting points.

in such systems, the upper end portion of the lift pipe usually extendsupwardly into a disengaging vessel of substantially greatercross-sectional area than that of the lift pipe, and terminates at alevel therein spaced from the upper end of the vessel a distancesufficient to effect the disengagement of the solids from the gas and tominimize attrition of the particles of contact material as a result ofimpingement against the confining walls or other internal surfaces ofthe disengager and of particle-to-particle impact as the contactmaterial falls to the bottom of the disengager or to the surfaces ofcollecting bafiles or trays which may be provided for the purpose ofminimizing the distance of free fall. A practicable disengaging heightbetween the upper end of the lift pipe and the top of the disengager hasbeen found to be up to about 20-25 feet.

It has been found that solids material may be pneumatically elevatedthrough a lift pipe at velocities which, if the solids were to bedischarged directly into the disengaging zone, would require adisengaging height for the solids substantially in excess of themaximum, and may be decelerated just prior to their introduction intothe disengaging zone by being passed through a decelerating pipe sectionof increased flow area at the upper end of the lift pipe.-

For this purpose, outwardly tapered decelerating pipe sections have beenemployed at the upper end of the lift pipe and have proved effective inreducing the discharge velocity of the solids stream by reason of thegradual increase in cross-sectional flow area of the lift path and theconsequent gradual expansion of the stream of lift gas.

It has been suggested that a cylindrical decelerating pipe section ofsubstantially greater flow area than that of the lift pipe be employed.Experience has shown, however, that to obtain any substantialdeceleration the difference in diameters of the lift pipe and thedecelerator pipe must not be too great, and the decelerator pipe must beof sufficient length to permit the decelerating factors to becomeeffective before the solids leave the decelerator pipe.

Furthermore, it has been demonstrated that during its passage throughthe cylindrical decelerating section the stream of solids has a tendencytoward non-uniform flow, accompanied by substantial recycling of thesolid parti cles. When the annular space between the lower end of thedecelerator pipe and the lift pipe is closed, such recycling causesflooding of the decelerator pipe with consequent spill-over of the solidparticles into the path of the stream emerging from the upper end of thelift pipe.

.Heretofore, the above-mentioned considerations imatent O "ice posedserious limitations on the practical velocities at which the contactmaterial could be discharged from the lift pipe.

In accordance with the present invention the discharge velocity of thecontact material from the upper end of the elongated lift pipe ismaintained at a desirable maximum, and the velocity of the contactmaterial as it actually discharges into the disengaging zone is reducedto a desirable low value, by passing the stream of contact material andlift gas from the upper end of the lift pipe into a relatively shortpipe of substantially greater diameter than the diameter of the liftpipe, whose length nevertheless is suflicient to effect a substantialdeceleration of the particles of contact material passing through it.The stream of lift gas and contact material thereafter discharges at asubstantially reduced velocity from the decelerator pipe into thedisengaging zone. In order to adjustably control the rate of suchdeceleration as the particles pass through the enlarged deceleratorpipe, additional lift gas is introduced in relatively minor butcontrolled amount into the lower end of the decelerator pipe. Suchadditional lift gas is introduced at the lower end of the enlargeddecelerator pipe in a manner to cause the additional gas to travelupwardly as an annular stream about the upper end portion of theelongated lift pipe. The additional lift gas is introduced in suchamount as to combine with the primary lift gas and maintain a relativelysmooth flow of contact material through the decelerator pipe. In orderto maintain the desired characteristics of contact material flow, therate of "introduction of the additional gas into the decelerator pipemay be regulated as desired, such as in response to pressure changesbetween vertically-spaced points along the decelerator path.

For a clearer understanding of the invention, reference may be had tothe following description and claims taken in connection with theaccompanying drawing forming a part of this application in which:

Fig. 1 is a diagrammatic elevational view showing a cyclic hydrocarbonconversion system employing a pneumatic lift for elevating the granularcontact material along the upfiow portion of its path of circulation, towhich lift the method and apparatus of the present invention mayadvantageously be applied; and

Fig. 2 is a fragmentary elevational view, in cross section, showing thedecelerator and the disengager constituting the upper end of thepneumatic lift.

Referring to Fig. 1 of the drawing, the numeral 11 indicates a typicalhydrocarbon conversion unit comprising a combination superimposedreacto'r-regenerator, such as that disclosed in an article entitledHoudriflow: New Design in Catalytic Cracking, appearing at page 78 ofthe January 13, 1949 issue of the Oil and Gas Journal.

Since the reactor-reg'enerator and its associated conduits for supplyingthe hydrocarbon charge, air, steam, etc., and for removing the gaseousproducts of conversion and regeneration form no part of the presentinvention, a detailed description thereof is omitted for the sake ofbrevity.

The pneumatic lift employed in conjunction with the combinationreactor-regenerator for the purpose of maintaining a continuouscirculation of the granular contact material is generally indicated bythe numeral 12. The lift comprises: an introduction chamber or engager13, located laterally below the lower end of the conversion unit 11,wherein the regenerated contact material conveyor thereto through sealleg 14 is engaged by a gaseous lift medium introduced through inletconduit 15; an elongated lift pipe 16 extending upwardly from a lowpoint within the engager to a level adjacent the upper end of the unit11; a decelerator pipe 17, wherein a controlled reduction of thevelocity of the contact material is eifected; and a disengager 18surrounding the upper end of the decelerator pipe 17, wherein thecontact material is disengaged from the lift gas, the lift gas beingdischarged from the upper end of the d1sengaging zone through outletconduit 19, and the contact material being discharged from the lower endthereof and conveyed to the upper end of the conversion unit 11 throughconduit 20, which conduit may if desired serve as a seal leg.

Referring to Fig. 2, the upper end of the lift pipe 16 extends arelatively short distance axially into the lower end of the deceleratorpipe 17. The decelerator pipe is of larger diameter than the lift pipe,thereby providing an annular space 21 between the upper end of pipe 16and the lower end of pipe 17. The lower end of annular space 21 isclosed off by means of an annular plate 22 mounted on the lift pipe andsecured to a flange 23 formed on the lower end of the decelerator pipe.A gas inlet conduit 24 is provided in the side wall of the deceleratorpipe near its lower end, so that additional lift gas may be introducedinto the lower region of the annular space 21.

The annular space 21 is of sufficient length to provide asmooth-flowing, upwardly-directed annular stream of additional lift gasrising about the periphery of the stream of contact material and liftgas discharging from the upper end of the lift pipe, as indicated by thearrows. The amount of additional lift gas so introduced is controlled bya valve 25 in the inlet conduit 24. The length of the path through thedecelerator pipe 17, from the upper end of the lift pipe 16 to thedischarge end of the decelerator pipe, is sufficient to effect asubstantial reduction in the velocity of the contact material, so thatupon discharge into the disengager vessel 18 it will require a minimumof disengaging height. That is, the required distance, between thedischarge end of the decelerator pipe and the top of the disengager, forsubstantially complete deceleration of the particles of contact materialmay be held to a minimum.

Operating in accordance with the invention, the contact material isengaged within the introduction chamber or lift engager 13 by primarylift gas in an amount sufficient to elevate the contact material throughlift pipe 16 at relatively high velocities. For example, by the time itreaches the upper end of the lift pipe the contact material may havebeen accelerated to a discharge velocity of up to about 70 ft./sec. Themost practical discharge velocity from the lift pipe will of coursedepend to some extent upon the characteristics of the particular contactmaterial, especially with respect to hardness.

As the lift gas and the contact material discharge into the largerdecelerator pipe, there is substantially immediate expansion of the liftgas stream, with a consequent reduction in velocity. The momentum of theparticles of contact material carries them a substantial distance withinthe decelerator pipe before there is any appreciable reduction invelocity. Beyond such point, however, the particles of contact materialdecelerate rapidly, so that by the time they reach the discharge end ofthe delecerator pipe 17 they have attained a relatively low velocity.

In order to insure that the contact material will not slug in passingthrough the decelerator pipe 17, additional or decelerator lift gas isintroduced through conduit 24 into the annular space 21 at the lower endof the decelerator pipe. It has been observed that without theintroduction of such decelerator lift gas, contact material tended toaccumulate in the annular space 21. When the accumulation of particlesreached a level above the discharge end of the lift pipe 16 theparticles avalanched into the rising high velocity stream dischargingfrom the lift pipe, with consequent high attrition of the particles.

Since the addition of lift gas into the decelerator pipe 17 tends tocounteract the velocity reduction effect resulting from the differencein flow area between the lift pipe and the enlarged decelerator pipe, itfollows that the maximum practical velocity reduction is effected whenthe amount of additional lift gas is held to a minimum consistent withsmooth operation of the lift. It is therefore contemplated that theintroduction of such additional lift gas will be in controlled amount,just sufficient to prevent undesirable slugging in the decelerator pipe.

To this end I may provide automatic means for controlling valve 25 inaccordance with changes in flow within the decelerator pipe. Forexample, such control means may comprise a differential pressurecontroller 26, diagrammatically illustrated in Fig. 2, connected betweenpressure taps 27 and 23 located at spaced points along the side of thedecelerator pipe. Differential pressure controller 26 is connected tovalve 25 by conduit 29 and is arranged to provide an increase in theflow of additional or decelerator gas whenever there is an appreciableincrease in pressure between pressure points 27 and 28,

such as would occur when the contact material begins to slug. Suchincrease in decelerator gas flow should be sufficient to clear thedecelerator pipe and restore the desired smooth flow of contactmaterial. It has been found that, in any case, there will be a netreduction in the discharge velocity of the contact material.

At the substantially reduced velocity, the particles discharge upwardlyfrom the decelerator pipe 17 as an unconfined stream, and thereafterdecelerate by force of gravity. The particles then fall freely to thelower region of the disengager 18, where they may be retained as acompact moving bed 30 to provide a surge capacity for the system, orwhere they may be drawn off immediately. In either case, the particlesare passed as a continuous stream through conduit 20 to the upper end ofthe unit 11.

The present invention is especially advantageous in those instanceswhere, by reason of the consequent lower pressure drop through the liftpipe for a given mass flow rate, it is desired to operate at highmaximum attained velocity within the lift pipe.

In present practice, it is a basic concept of pneumatic lift operationthat the rate of solids flow through the system is best controlled atthe lower end of the lift pipe, the manner of introduction and theamount of lift gas introduced being controlling factors. Velocityreduction at the upper end of the lift in accordance with this inventionis consistent with such concept.

When operating at a high discharge velocity from the lift pipe, that is,a high maximum attained velocity, a straight cylindrical deceleratorpipe section requires a relatively short vertical distance to achievethe desired velocity reduction. Any tendency toward slugging orrecycling of the solid particles while passing through the deceleratorsection is overcome by the introduction of the auxiliary gas at the baseof the decelerator pipe. Such gas is introduced primarily for thepurpose of maintaining a smooth flow through the decelerator.

The need for such auxiliary gas introduction was demonstrated in anexperiment with a 12-inch diameter lift pipe, fitted with a 20-inchdiameter decelerator pipe. The discharge end of the latter was located20 feet above the discharge end of the lift pipe. A series of catalystruns were made under various conditions, and the height of rise of thedischarged catalyst within the disengager was determined. Initially, thecatalyst was elevated without the use of decelerator air. In thoseoperations it was found that the annulus between the l2-inch and the 20-inch pipes rapidly filled with catalyst. The level of this catalysttended to build up as a compact bed in the 20- inch pipe to more thanone foot above the top of the l2-inch pipe. When catalyst built up inthis fashion, additional catalyst falling back on top of this bed fromthe upper region of the disengager would slide intermittently into therising high-velocity catalyst stream. A continuous heavy recycle ofcatalyst was also observed adjacent the wall surfaces of the deceleratorpipe at a level about one third of the distance along the deceleratorpath. The introduction of decelerator air eliminated the catalystbuild-up in the decelerator pipe, and increasing amounts of theadditional decelerator air decreased the amount and density of recyclingin the decelerator pipe.

This is not to say, however, that such auxiliary decelerator gas doesnot effect some measure of control upon the solids height of rise. Forexample, in an air lift comprising a l2-inch diameter lift pipe toppedby a 20-inch diameter cylindrical decelerator pipe forming a 20-footextension of the confined lift path, it was demonstrated experimentallythat with a lift engager air rate of about 3000 cu. ft./min. and aconstant decelerator rate of about 1000 cu. ft./min. a reduction of cu.ft./min. in the engager air rate reduced the solids height of rise byabout four feet; whereas, with the same initial air rates, incrementalreductions of 500 cu. ft./min.

' in the decelerator air rate reduced the solids height of rise by /2 to1 /2 feet for each increment.

In a further experiment, with a 12-inch diameter lift pipe dischargingsolids at about 35 ft./sec. into a disengager vessel having adisengaging distance of 20 ft. between its upper end and the dischargeend of the lift pipe, it was observed that a substantial portion of thesolids forcefully impinged against the upper end of the vessel. When a20-inch diameter-ZO-foot decelerator pipe was installed and operated inaccordance with the invention, that is, with auxiliary gas introductionsufficient only to achieve a smooth solids flow, the height of rise ofthe particles decreased to about 1314 feet, which is within thepracticable range of disengaging height.

While decelerator air rates in excess of 25% of the lift engager airrate do not have an appreciable effect on the height of rise when thelift stream is expanded from a 12" diameter lift path to a 20" diameterdecelerator path, it is believed that a smaller differential between therespective flow path areas will show a substantially greater effect forthe decelerator air. Accordingly, in cases where it is not desired tocontrol the height of rise by adjustment of the lift engager air rate,the decelerator air rate may be made the controlling factor by providinga smaller flow path expansion in passing from the lift pipe into thedecelerator pipe.

Operation in accordance with the present invention permits the use ofrelatively high discharge velocities in the lift pipe, and effectivelyreduces the velocity of the catalyst stream before it discharges intothe disengaging zone. The substantial reduction in height of riseeffected by the decelerator pipe permits a reduction in the height ofthe disengager vessel, thereby producing substantial savings in the costof materials and construction.

The invention provides additional operational advantages, especiallywhen starting up a unit. For example, before catalyst circulationisstarted, enough decelerator air is turned on to insure that catalystwill not slug in the decelerator pipe. When catalyst circulation isestablished, the decelerator air may be cut back to obtain the desiredexit velocity for catalyst leaving the decelerator, which exit velocitymay be determined by means of a height of rise determination in thedisengager.

While the invention may be practiced to advantage through a relativelywide range to size differences in the cross-sectional flow areas of thelift pipe and the decelerator pipe, for most practical purposes the flowarea of the decelerator pipe will be in the range of 1 /2 to 2 /2 timesthe flow area of the lift pipe.

With respect to the decelerator pipe, its most practical size will befound when the ratio of length to diameter is in the range of between 7to 1 and 18 to 1.

Obviously many modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated in the appended claims.

What is claimed is:

1. In a pneumatic lift system for circulating granular materialincluding an elongated vertical lift pipe and a disengaging chamber atthe upper end of said lift pipe, the combination therewith of asecondary elongated lift pipe, a relatively short decelerator pipe ofgreater crosssectional flow area than said lift pipe and forming anupper longitudinal extension of the latter, the upper end portion ofsaid lift pipe extending a distance axially into the lower end portionof said decelerator pipe and the upper end portion of said deceleratorpipe extending a distance into said disengaging chamber, means forclosing the lower end of the annular space formed between said lift pipeand said decelerator pipe, and means for introducing controlled amountsof lift gas into the lower region of said annular space.

2. Apparatus as defined in claim 1, including means for controlling theamount of said lift gas introduced into the lower region of said annularspace in accordance with a differential pressure betweenvertically-spaced points along the lift path formed by said deceleratorpipe.

3. In a pneumatic lift system for circulating granular materialincluding an elongated vertical lift pipe and a disengaging chamber atthe upper end of said lift pipe, the combination therewith of arelatively short decelerator pipe of greater cross-sectional flow areathan said lift pipe and forming an upper longitudinal extension thereof,the upper end portion of said lift pipe extending a distance axiallyinto the lower end portion of said decelerator pipe to form an annularspace therebetween, and the upper end portion of said decelerator pipeextending a distance into said disengaging chamber to provide adisengaged granular material collecting space at the bottom thereof,means for closing the lower end of said annular space formed betweensaid lift pipe and said decelerator pipe, and controllable means forintroducing lift gas into said annular space at a distance from itsupper end sufficient to assure a smooth upward flow of said lift gasabout the discharge end of said lift pipe.

4. In a pneumatic lift system for circulating granular materialincluding an elongated vertical lift pipe and a disengaging chamber atthe upper end of said lift pipe, the combination therewith of arelatively short decelerator pipe of greater cross-sectional flow areathan said lift pipe and forming an upper longitudinal extension thereof,the upper end portion of said lift pipe extending a distance axiallyinto the lower end portion of said decelerator pipe to form an annularspace therebetween, and the upper end portion of said decelerator pipeextending a distance into said disengaging chamber to provide adisengaged granular material collecting space at the bottom thereof,means for closing the lower end of said annular space formed betweensaid lift pipe and said decelerator pipe, and controllable means forintroducing from within said annular space an upwardlydirectedsmooth-flowing annular stream of lift gas about the upper periphery ofsaid lift pipe.

5. Apparatus as defined in claim 4, including means for controlling theamount of said lift gas comprising said annular stream in accordancewith a differential pressure between vertically-spaced points along theenlarged extended portion of said lift pipe formed by said deceleratorpipe.

6. Apparatus as defined in claim 4, characterized in that thecross-sectional flow area of said decelerator pipe is in the range 1 /2to 2% times the cross-sectional flow area of said lift pipe.

7. Apparatus as defined in claim 4, characterized in that the ratio oflength to diameter of said decelerator pipe is in the range of about 7/1 to 18/1.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,435,927 Manning Feb. 10, 1948 2,460,546 Stephanoff Feb. 1,1949 FOREIGN PATENTS Number Country Date 258,524 Italy Dec. 8, 1925

